@Abedeus By this point, I'm pretty sure everyone would know what a "Maou" is, at most a brief translator note would be necessary to explain the Magic King/Demon King readings. Actually, even with the current translation such a note would be necessary because otherwise people won't get why people are calling him Demon King all of a sudden.
Honestly, I feel like I'm missing something because the whole different readings jokes make no fucking sense if they aren't speaking japanese or a language that is really similar. Unless Shin has some auto-translation skill that I forgot about.
It's not really that much of an issue but it looks really weird when the translation has everyone say "Demon King/Lord" when there are actual demons in the setting (what japanese term do they use for them, by the way?) and no one seems to find this weird other than Shin. It also makes no sense because why would they call the magician leading the fight against the demons a Demon King?
My personal suggestion is to either leave it as just "Maou" and explain the reading thing in a TN or to use "Magic King" which seems to make more sense and allows Shin's reaction to the title to be handwaved as him associating Magic Kings with Demon Kings or evil overlords in general.